Manufacture of sulphite pulp



Oct. 2, 1945.

G. H. TOMLINSON MANUFACTURE OF SULPHITE PULP Filed April 8, 1941 4Sheets-Sheet l as@ ESE Oct. 2, 1945. G. H. ToMLlNsoN MANUFACTURE OFSULPHITE PULP Filed Apr-11 8 1941 4 Sheets-Sheet 2 Oct. 2, 1945. GTOMLINSQN 2,385,955

MANUFACTURE 0F SULPHITE PULP Filed April 8, 1941 4 Sheefcs-Sheet I5 .ITFll' ATTORNEY.

Oct. 2, 1945. G. H. ToMLlNsoN MANUFACTURE OF SULPHITE PULP 4'sheets-sheet 4 Filed April 8, 1941 MAX/MUM HMPERA TURF 0F ASH "/f.

1N VENTOR. @6o/ge H, Tm/'nsdn ATTORNEY.

Patented Oct. 2,y 1117117945 RLIANUFACTURE F SULPHITE PULP George H.Tomlinson, Westmount, Quebec, Canada Application April 8, 1941, SerialNo. 387,474

13 Claims.

The general object of my invention is the proi vision of an improvedsystem of manufacturing pulp from cellulosic fibrous material using arelatively pure magnesium base sulphite cooking liquor. A further objectis the provision of an improved self-supporting cyclic system ofrecovering chemicals and heat from, and completely disposing of, theresidual pulp liquor in a process of the character described. A furtherand more specic object is the provision of an improved process oftreating the residual pulp liquor to recover magnesium and sulphur in aform permitting their economic reuse in the pulping process, and heat'ineconomic quantities.

'Ihe various features of novelty which characterize my invention arepointed out with partlcularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which I have illustrated and described a preferred embodimentof my invention.

Of the drawings:

Fig. 1 is a diagrammatic flow diagram of a cyclic sulphite pulp liquorrecovery system embodying my invention;

Fig. 2 is a view similar to Fig. 1 in the form of a material balanceshowing the daily quantities involved in a plant having a production of100 tons of pulp per day;

Fig. 3 is an enlarged sectional elevation of the recovery'furnace andboiler shown in Figs. 1 and 2: and

Fig. 4 is a curve sheet showing the relation between time, temperatureand mean reactivity of the ash produced.

The cyclic recovery system shown in Fig. 1 is of the general characterdisclosed in my prior applications Serial No. 186,938, filed January 26,1938, now U. S. Patent No. 2,285,876, and Serial No. 221,304, filed July26, 1938, now U. S.,Patent No..2,238,456, of which this application is acontinuation-in-part. InA such cyclic systems a cooking liquorconsisting of a relatively pure acid g sulphite compound of magnesium,i. e. magnesium bisulphite, with an excess of sulphur dioxide, issupplied toa digester I0 from a cooking acid tank II. 'I'he digester I0is shown as being equipped for indirect heating, permitting the Baume ofthe residual liquor following cooking to bemaintained at a higher valuethan would be possible if direct steaming were employed. A moreeconomical evaporation of the liquor to the desired concentration forcombustion is thus made possible.`

fWhen the cooking operation is completed the contents of the digesterare discharged into a blow pit I2 from which gases are vented and thepulp and liquor pumped to suitable pulp washing equipment, such asrotary vacuum iilters I3 and I4 arranged in series.

Wash water from a hot water tank I5 is delivered to the pulp washer I4as indicated and the filtrate discharged into a tank I6, from which aportion is delivered to the pulp washer I3 to serve as wash liquortherein. The filtrate from the second washer I4 is also employed toinsure the correct consistency of the pulp going to that washer byreturning a portion of the filtrate to the stock box of the washer. Afurther advantageous use of the ltrate is as a basis for the magnesiasuspension employed in the absorption towers, even though this resultsin the circulation of a small amount of inert solids in solution in thetotal system, as the filtrate thus forms the basis of subsequent cookingliquor. The lltrate from the washer I3 is sent to an acid Waste liquortank Il from which a portion is returned to the stock box of the washerI3, and another portion to the blow pit IZ to increase the fluidity ofthe pulp and thus facilitate its delivery to the pulp washers.Substantially all of the liquor and washer filtrate is thus maintainedin the system, the only loss being that carried out by the pulp leavingthe second washer I4 for subsequent treatment in the knotters, riiliers.bleachers and dryers.

The residual liquor in the tank Il will have a solid content ofapproximately 10%. This liquor is advantageously utilized as the washliquor in a gas scrubber or spray tower I8 which receives the heatinggases generated in the chemical recovery furnace, after a major portionof the solid chemicals have been separated therefrom, and prior to theabsorption towers. 'I'he spraying of the liquor into intimate contactwith the hot gases effects the liberation of free sulphur dioxide in theliquor which upon its liberation is carried with the heating gases forrecovery in the subsequent absorption towers. It also results in therecovery of most of the small amount of solid chemicals present in thegases coming to the scrubber, and the magnesium oxide so recovered isused for the partial neutralization oi' the acid liquor. The amount ofash present in the gases can be controlled by varying the operation ofthe cyclones. Some evaporation of the'weak-acid liquor also results,reducing the amount of concentrating to be done in the evaporators andthus saving steam.

The partly neutralized and partly concentrated residual liquor is thenpumped from the scrubber and the neutralization is then completed by thecontrolled addition of magnesium oxide to a neutralizing tank i9 anddelivered to the neutral waste liquor tank 20. The neutralization iseither completely or very largely accomplished by the use of recoveredash, but as indicated in Fig. 1, if magnesia is used as make-up to thesystem, the fresh magnesia may be introduced at this point. If magnesiumsulphate is used as make-up to the system, this is preferably added tothe liquor following its concentration, so that the inert sulphate willbe reduced to highly reactive magnesium oxide on passing through therecovery furnace, and thus become immediately eilective as activechemical. If the sulphate were added to the cyclicsystem subsequent tothe furnace, it would be an inert burden in the first batch of cookingliquor of which it formed a part. The acid residual liquor is highlycorrosive,vhaving a pH of 2.5 to 3.0, and unless neutralization is efefected the equipment handling the liquor must necessarily be made ofrelatively expensive corrosion-resisting alloy. Neutralization of theacid residual liquor prior to the delivery of the liquor to a multipleeil'ect evaporator is also important in that it eiects the combinationof the free SO2 present, thus preventing its liberation in theevaporator, a condition which would adversely aifect the vacuum. Theneutralizing magnesia dissolves readily, producing a slightly alkalineliquor hav ing a pH normally in the range of 7.2 to 8.5. Noprecipitation of organic or inorganic matey rial has been found to occureven at a pH of 8.5.

Further concentration ofthe neutralized residual liquor to aconcentration of 50-60 percent solids (3140 Baum) is economicallydesirable, and this is attained by passing the neutralized liquor fromthe tank 20 to a multiple effect evaporator system shown as a six-stageevaporator 2|, in the rst stages of which the liquor is heated byexhaust steam from a back pressure turbine Per cent Free moisture 45.0Carbon 25.04 Hydrogen 2.41` Sulphur f 2.751 Ash 6.87 Oxygen and nitrogen17.93

B. t. u. per 1b., 41.60.

The concentrated liquor is then delivered to a storagetank 23 andsupplied as required to the caustic magnesium oxide (MgO), and also morechemical recovery unit 2l and burned therein in suspension underself-sustaining combustion conditions to yield a dry ash consistingmainly of than enough steam to satisfy the requirements of the system.The construction and operation of the recovery unit is hereinafter setforth and is also disclosed and claimed in a co-pending application ofLeslie S. Wilcoxson, Serial No. 347,944, illed July 27, 1940, now U. S.Patent No. 2,354,175.

The ash produced by the spray burning of the liquor leaves the recoveryunit in suspension in the furnace gases which are then passed throughsuitable ash separating apparatus, such as a group of cyclone separators25 arranged in parallel, from which the separated ash is withdrawn anddelivered to an ash storage tank 26. 'I'he operation of the cyclones isparticularly effective as the chemical is normally present in the-gasesin particles of sulcient size to permit of their removal by suchseparating means. The conditions of the gases and chemicals when in thecyclones are such as to insure ease of operation and long life to thatapparatus.

An induced draft fan 21 is located at the sas discharge end of thecyclones. This is an advantageous location for the fan as the solids inthe gases have been reduced to a point suitable for goodfan operationand the temperature of the gases is suiliciently above the dew point ofthe .gases to definitely avoid any corrosive action on the contacted fanparts.

Most of the remaining ash in the gases is removed during the passage ofthe gases through the scrubber i8, as has been described. On leaving thescrubber the gases are delivered to serially connected absorption towers28 and 29 for the recovery of the sulphur content of the gases. In thesulphur absorption towers, the gases are subjected to contact with aslurry consisting mainly of magnesia in suspension formed by mixing theash from the tank 26 with the filtrate from the second pulp washer I4.The slurry is fed into the top of each absorption tower and passesdownwardly over wooden slats therein in counter-current relation to therelatively low temperature ascending gas. During its passage through thetowers the magnesia in suspension combines with the sulphur dioxidecontent of the gases and forms a solution of sulphited magnesium. Anysulphur trioxide present in the towers will combine to form magnesiumsulphate in the resulting liquor which relation while forming the inertsulphate, retains the chemicals in the system for subsequent reductionand reuse. By the foregoing operations the gases passing from the upperend of the absorption tower 29 to the stack have had both their solidmagnesium oxide and gaseous sulphur compounds eillciently removed andare consequently innocuous.

In order to have and maintain the greatest recovery efficiency in theabsorption towers, a balance is required between the amount of magnesiumoxide in the slurry and the amount of sulphur dioxide in the gases. Theaccumulation of calcium impurities in the system is controlled to causethe sulphited calcium to remain in suspension and the magnesium to gointo solution as sulphite and bisulphite. The slurry fed to the towersis preferably a liquor of high alkalinity, i. e.. a liquor having a pHvalue of about 9.5, but as an increasing amount of sulphur dioxide iscombined, the pH value falls, first gradually and then rapidly. Asdisclosed in my lsaid Patent No. 2,238,456, the pH value of the liquorleaving the .tion towers 29 and 28 is automatically controlled inresponse to variations in the percentage of SO2 in the gases passing tothe absorption towers, by means of an automatic SO2 analyzer 3| and arei mote control valve 32. A supplementary control temperature isreduced to facilitate the subsequent absorption of SO2 from the sulphurburners. The operation of the valve 32 in response to the SO2 analyzeris checked by having a pH control 33 at the discharge side of the cooler34 act on the valve 32, preferably between the operating intervals ofthe SO2 analyzer. An SO2 recorder 35 is also employed.

The necessity for clean cooking liquor, in view oi' the possibility ofsome solids being present in the washer ltrate andthe collection ofsolids in the form of unburned carbon from the gases in the absorptiontower, is provided for byiilterng the liquor in a suitable filter 40. Byhaving the lter subsequent to the pH control operation, the calciumcompounds in suspension in the liquor can be removed along with anycarbon or other solid particles. The magnesium bisulphite liquor isfortified to the desired sulphur dioxide concentration by bringingsulphur dioxide, generated by the sulphur burners 4l and cooled in thefgas cooler l2, into contact with the liquor while passing through a gasabsorption system 43. The fortifled liquor is then delivered to a tank44 where "t is mixed with relief gases from the digester I before beingdelivered to the cooking acid storage tank il.

The high cost of chemicals involved in a pulping process employing arelatively pure magnesium base cooking liquor requires a cyclic recoveryprocess having a high efficiency of recovery of the heat and chemicalvalues of the residual liquor to be economic. magnesium oxide, must alsobe recovered in a form which permits their economic reuse in thelpulplng process and the heat values in the liquor ysis being:

Per cent CO2 17.7 SO: 0.6 Oz 2.40

Any attempt to recover the sulphur dioxide from The chemicals, sulphurand `passed through a liquor cooler 34 inwhich its quently be useless.

the gases under such conditions is'unusual because its recovery can onlybe effected in the presence of an extremely reactive reagent. Anoverheated or "dead-burned magnesia-would conse- In fact, the magnesiashould preferably be more reactive than the best grades of commercialcaustic magnesia to effect the desired amount of sulphur dioxiderecovery under such conditions.

Magnesia is commercially produced by the calcination of magnesiumcompounds, such as magnesite, by coal or carbon mixed with the material.It is known that the reactive character of the resultant magnesiadepends upon the `amount of carbon present in the mixture; and it isgenerally considered that for the production of caustic magnesia, theamount of coal or carbon used should not exceed about 20% of the weightof the magnesite. For the commercial production of dead-burned magnesia,it is the customary practice to mix from 30% to 50% coal with themagnesite.

In the representative analysis of the concentrated liquor given, thecarbon is 25.04% and the ash (i. e. the magnesium compounds) 6.87%.Accordingly, if such liquor were evaporated to dryness before beingintroduced into a furnace and the dried solids then burned, the materialwould then include an amount of carbon over ten times the amount ofcarbon required for producing dead-burned magnesia. Complete combustionof the organic constituents of the residual liquor is highly desirable,both in the interests of thermal efficiency and to obtain an ash as freefrom impurities as possible in order to minimize the subsequentfiltering operation.

In accordance with my recovery process, pulp residual liquor of thecharacter described can be treated to obtain an ash containing a highpercentage of magnesia of a high reactivity and free from carbon whilemaintaining desirable combustion conditions in the recovery furnace. Ithas been found that recovery furnace conditions suitable for obtainingcertain of these characteristics are not suitable for obtaining others.For example, it is important that any magnesium compounds in the form ofmagnesium sulphate be reduced to magnesium oxide While in the furnace asthe sulphate is of no value in the cooking liquor and would form a deadload of chemical circulating through the system. The greatest reductionof the sulphate will occur when a high temperature reducing atmosphereis maintained in the furuace. However, high temperature :conditions arenot suitable for obtaining the highly reactive caustic magnesia desired,nor isa highly reducing atmosphere suitable for a complete combustion ofthe combustible portions of the liquor. Complete combustion of thecombustible ymatter is favored by the use of a substantial amount of.excess air in the furnace, but such conditions tend to increase thepercentage of sulphur dioxide (SO2) converted into sulphur trioxide(S03), this reaction being accelerated in the temperature range(l000-l200 F). While any sulphur trioxide in the heating gases can berecovered in tl'e absorption towers, it would combine with the magnesiain the slurry to form magnesium sulphate and add to the dead load ofcirculating chemicals. The production 'of caustic magnesia of thedesired reactivity, i. a mean reactivity at least greater than 1.5 on anarbitrary scale which has been devised, on whichl ordinary commercialcaustic magnesia has a mean reactivity of 2.1, requires careful controltherein.

of the temperature and atmosphere conditions, velocityof gas flow in thefurnace, and size of the ash particles. It has been found that furnacetemperatures in the range of l8002400 F., and preferably 21002300 F.,are most desirable with a rapid passage of the ash particles through thefurnace sections having such temperatures, the permissible time ofexposure decreasing as the temperature increases. If the exposure atthese temperatures is more than momentary, the reactivity of themagnesia diminishes. and even dead burning is likely to occur. Therelation of the ash solubility in acid relative to the time andtemperature of exposure is illustrated in Fig. 4, in which the uppersolid curve is for a one second exposure and the lower broken line curvefor a two second exposure. All of these factors require consideration inthe construction and operation of the recovery apparatus.

As shown in Fig. 3 the heat and chemical recovery apparatus comprises acombined furnace and steam boiler unit 24 having a furnace chamber 50 ofrectangular horizontal and vertical cross-section formed by refractorywalls consisting of a vertical front wall 5I, side walls 52,roof 53,floor 54, and bridge wall 55. A rectangularly shaped gas passage 56connects the furnace chamber 50 with a vertically elongated narrowpassage 51 at the rear side of the bridge wall 55, constituting anunobstructed open passbetween the chamber 50 and the convection heatabsorbing section of the unit.

At the rear side of the passage 51 is arranged a bank of verticallydisposed steam generating tubes 60 connected to a steam and water drum6I and a lower water drum 62. The front row of tubes 60a are offsetlaterally and provided with integral metallic studs over the greaterportion of their length with the intertube spaces filled with refractoryto form a water cooled baille 63 extending downwardly from a pointadjacent the drum 6| and defining the rear'wall of the passage 51. Theremaining unstudded portions of alternate tubes 60B are bent in spacedrelation to form a water tube screen 64 across the gas discharge openingat the rear side of the passage 51. A horizontally extending refractorybaiile 65 forms the bottom of the gas passes within the tube bank. 'Iherear portion of the baffle 65 slopes downwardly to form one side of ahopper 65a having a bottom outlet for ash deposite A vertical refractorybaille 66 terminates short of a horizontal refractory extension 63a ofthe tube baille 63. Another vertical refractory baffle 61 extendsdownwardly from the baille extension 63 to form a vertical gas pass 68with a gas exit 69 at its lower end. Most of the steam generating tubes60 are positioned within the gas pass 68. The two rearmost rows of tubes60,are positioned between the baille 61 and the adjacent wall of theassociated air heater. Access doors 50a and 51a are provided at thebottom of the various gas passages where ash may tend to collect.

The bailles 63 and 66 define a vertical gas flow passage 10 ofsubstantially the same width as the gas passages 51 and 68. The space 10is divided into three sections transversely of the unit by transverselyspaced groups of vertical steam generating tubes li0D arranged inalignment between Ithe bailles 63 and 66. The tubes of each group havetheir lower ends connected to the water drum 62 and their upper endsconnected to a corresponding short horizontal header 1l, connected tothe drum 6l by tubes 12.

With the foregoing construction the heating gases leaving the lower endof the passage 51wil1 pass through the tube screen 64 into the lower endof the sections of the passage 10, all of the walls of which are thusdefined 4by Water tubes. The temperature of the gases therein is rapidlyreduced by radiation to and contact with the surrounding water tubes.The quenched condition of the heating gases in the passage 10 permitsthe installation of superheating surface in the upper portions of thevsections thereof, and, as shown, the superheating surface consists ofsmall diameter multi-looped tubes 13 arranged in parallel vertical fiatcoils and connected to superheater inlet and outlet headers 15 and 16respectively. Tubes 11 connect the inlet header 15 to the steam space ofthe drum 6l.

The concentrated liquor from the liquor storage tank 23 is delivered bya pump 80 through a pipe line 8| t having a. return flow connection 62to the tank) to the furnace. Separately controllable liquor nozzles 83positioned in corresponding burner ports 84 in the furnace roof 53 areemployed for introducing the liquor. An atomizing steam nozzle 85alongside each liquor nozzle 83 is supplied with steam from the turbine22, as indicated in Fig. l. Each pair of liquor and steam nozzles has anassociated adjustable distributor block 86 carried by a. bracketsupported on the nozzle assembly. Each block has a vertically arrangedimpact surface, against which the steam jet impacts and is deectedacross the converging liquor stream, breaking up the liquor stream intoa relatively flat finely divided spray distributed substantially in aplane parallel and adjacent to the front wall of the furnace.

Combustion air is separately supplied to each burner port from apreheated air duct 90 by branch ducts 9| controlled by dampers 92. Thedu'ct 90 forms a branch of a main air duct 93 connected to the outlet ofa tubular air heater 94 at the rear of the boiler bank. A forced draftfan |05 maintains a pressure air supply to the air heater and furnace,the fans 21 and |05 being manually or automatically regulated tomaintain the desired furnace pressure and gas velocity throughout theunit.

In operation the furnace chamber 50 is initially heated to apredetermined temperature by auxiliary fuel, such as a wood fire on thefurnace bottom 54 or auxiliary oil or gas burners temporarily insertedinto the furnace. With the burner arrangement described the residualliquor introduced will burn in suspension while in a verticallyelongated U-shaped path extending downwardly along the front wall 5I,across the furnace floor, and upwardly along the bridge wall 55 to thegas exit 56. The burning fuel particles and products of combustion thenpass downwardly 4through the first open pass 51, and through the tubescreen 64 into the various sections of the passage 10. The gas streampasses upwardly through the sections of the passage 10 in contact withthe water tubes forming the walls thereof in an unobstructed flow untilreaching thesuperheater tubes 13. The gases then flow along thesuperheater tubes over the upper end of the baile 66 and downwardlythrough the main generating section of the boiler longitudinally of thetubes 60. 'I'he gases pass across the lower egidtssosf the downcomertubes |50c and out the gas e To permit the eiilcient recovery ofmagnesium and sulphur in a form permitting their economic reuse in thecyclic system, introduction of the chemicals inw the furnace in aconcentrated liquor of the character described and maintenance ofpredetermined temperatures, atmospheres and gas velocities in differentportions of the unit are of prime importance. A long flame combustion ofthe combustible constituents of the residual liquor under a reducingatmosphere while in intimate contact with the magnesium constituents toa location adjacent the bottom of the first open pass is considereddesirable to avoid excessive furnace temperatures and possibledead-burning of the magnesia. For this purpose the total amount cicombustion air supplied to the furnace is only slightly in excess ofthe.

theoretical combustion requirements and that air is supplied to thefurnace at widely spaced points along the flow path in predeterminedproportions.

A total combustion air approximately 110% of the theoreticalrequirements has been found suitable, for example. About 80% of thetheoretical amount is introduced through the conduits 9| and burnerports 04 along with the sprayedliquor to be incinerated. The reverbatoryeil'ect of the U-shaped flame path in the furnace facilitates the dryizdistillation, ignition and combustion of the entering liquor.

'I'he liquor particles are burned in suspension as they pass downwardlyalong the front wall 5| and a considerable portion of the chemical'ashtends to separate from the burning fuel stream as the flame and gasstream turns acrossr the floor 54 and upwardly towards the gas exit 56.The

4separated chemical tends to deposit on the floor 50 and if Apermittedto so deposit and remain thereon would rapidly become dead-burned andsubstantially useless in the pulping process. A second supply ofcombustion air is delivered to the furnace from a main l branch duct 90through horizontally arranged inlet passages 85 formed along the frontwall 5| at the level of the furnace floor 54 and opening into a branchair duct 98 controlled by a damper 91. Normally, about 15% of thetheoretical air supply is discharged by the inlets 05 horizontallyacross the furnace floor 54 to supply additional air for combustion andalso cause any ash tending to deposit on the furnace door to be sweptout of the furnace with the furnace gases. With the described amounts ofcombustion air supplied, a reducing atmosphere is maintained throughoutthe furnace chamber 50 and open pass 51, and after probably a break-downof the magnesium lignin sulphonate to magnesium sulphate, the magnesiumsulphate is reduced to magnesium oxide with the release of sulphurdioxide and carbon dioxide approximately in accordance with the formula:

'I'he thick refractory walls of the furnace chamber with theirsubstantial heat storage capacity contribute to the maintenance ofuniform temperature conditions therein. The heating gases and suspendedash particles passing out through the gas exit 56 down through thepassage 51 contain some unburned carbon particles and combustible gases,the combustion of which is completed in the lower part of the passage 51and passage l0 by the introduction of a third air supply through aseries of air inlet openings 98 opening through the rear side of thebridge wall l5. The air inlets 98 are connected to an air duct 09,having a control damper |00, through vertical passages |0| in the bridgewall and horizontal passages |02 below the furnace floor. About 15% ofthe theoretical air supply is delivered to the ports 00 and an oxidizingatmosphere thus maintained in the lower part of the passages l1 and l0.Combustion is completed and the temperature of the heating gases andsuspended ash particles is rapidly reduced as the stream enters thepassage 10 by radiation to the water cooled walls of that space and thesubdividing tube groups 0b. The lowering of the temperature of the gasesaffords a safe metal temperature for the superheater tubes 13.

The furnace is thus designed with respect to shape and air admission tomaintain a reducing atmosphere of gases in that portion of their travelto a location in the first open pass l1, where additional air isadmitted to complete com.. bustion just prior to the gasventrance to thesecond open pass 10. In contrast to the mainly refractory constructionof the first pass walls, the walls of the second pass are mainly watercooled. The time-temperature relation necessary to obtain the resultingmagnesium oxide ash in a highly reactive condition best suited for thesubsequent production of magnesium bisulphitecooking acid is thusobtained. While the addition of air and the production of an oxidizingatmosphere in the lower part of the first open pass insures completionof combustion, the reaction of the sulphur dioxide released to sulphurtrioxide is minimized, even with the oxidizing atmosphere present, bypassing the gases quickly through the temperature range (10001200 F.) inwhich this reaction is accelerated, by having the gases on completion ofcombustion enter the main tube bank pass 68 wherein the gas temperatureis rapidly reduced through this range.

The chemical recovery unit described when used to treat the residualliquor from a pulp mill having a pulp capacity of 100 tons per day andwith liquor of the analysis given, will receive approximately 19,3001bs./hr. With the furnace construction, and distribution and gasvelocity described herein, the gas temperature on leaving the furnacechamber 50 will be approximately 2250 F. and entering the screen 84approximately 2l40 F.

The ash produced will be mainly in the form of light cenospheres andflakes having a density of from three to eight lbs/cu. ft., the densityincreasing as the reactivity decreases. 'I'he percentage of causticmagnesium oxide in the ash will depend upon the efficiency of reduction.Normally the caustic magnesium oxide content will be at least 70%, withthe remainder mainly magnesium sulphate and carbonate. Very smallpercentages of inorganic impurities will also be present, these varyingwith the wood used.

The boiler heating surface is characterized by its simplicity ofconstruction and ease in which it can be kept absolutely clean of ash.'I'he furnace temperatures maintained are below the fusion temperatureof the ash, and the tubes and ballies of the boiler are so arranged thatany of the dry ash depositing thereon can be readily removed. All of theboiler tubes are vertical as well as the ballles defining the boilerpasses. Both of the horizontal baflies 63 and 05 are readily cieanableand insure freedom of corrosion of the drum tube connections from thehigh sulphur 6 assauts gases would cause corrosion. The ability tomaintain the boiler heating surface absolutely cle'an minimizes thedraft loss and permits the use of high gas velocities, such as 50 feetper second, for example, consistent with the economical utilization ofinduced draft fan power requirements and consequently results in highlyefficient heat transfer conditions. The rapid travel of the gases andsuspended chemicals at such velocities is indicated by the fact that thelength of the flow path from the spray nozzles to the bottom of theiirst open pass may be approximately 50 feet, for

- example.

On leaving the boiler, the heating gases pass upwardly, through thetubes of the air heater with the air to be preheated flowing downwardlyaround the tubes under the action of a forced draft fan |05. The gasesthen flow in parallel through the dust collecting cyclones forming theseparating apparatus 25. Most of the chemical ash in suspension isseparated at this point and collected in hoppers at the bottom of thecyclones. The gases then pass out through the induced draft fan 21 tothe spray tower I8. 'Ihe spray tower I8 is divided into a pair of narrowand wide passes. The gases flow downwardly through the narrow passage inwhich they successively contact descending sprays of residual liquordelivered from the tank I1 to vertically spaced spray nozzles. The gasesthen flow upwardly through the wide pass to the absorption towers 28 and29. A pump receives the liquor from the bottom of the spray tower anddischarges it to the neutralizing tank.

While in accordance with the provisions of the statutes I haveillustrated and describedvherein the best form of the invention nowknown to me, those skilled in the art will understand that changes maybev made without departing from the spirit of the invention covered bymy claims,

and that certain features of the invention mayy sometimes be used toadvantage without a corresponding use of other features.

I claim:

1. The method of treating the residual liquor resulting from thedigestion of cellulosic fibrous material in a relatively pure magnesiumbase sulphite cooking liquor and separation from the pulp in a pulpwashing system which comprises concentrating the liquor by evaporation,burning the combustible organic constituents of the concentrated liquorin a. furnace chamber in suspension therein under temperature conditionsand for an interval suiiiciently brief to obtain a dry ash having a highpercentage of caustic magnesia and combustion gases containing a lowpercentage of sulphur dioxide and substantially all of the ash insuspension therein, separating ash from the combustion gases, thenpassing unconcentrated liquor into intimate contact with the combustiongases for the recovery of additional ash from the gases and partialevaporation and neutralization of the unconcentrated liquor, completingthe neutralization of the partially neutralized and evaporated liquor bythe addition of caustic magnesia thereto prior to further evaporationthereof, mixing recovered ash with pulp washer filtrate to form analkaline aqueous suspension, and passing the ash suspension through agas absorption chamber in contact with the combustion gases to recoversulphur dioxide.

2. In the method of making pulp by the digestion of cellulosic fibrousmaterial in a relatively pure magnesium base sulphite cooking liquor,the steps which comprise passing the cooked pulp through a multi-stagepulp washing system, concentrating the liquor from the ilrst washingstage to a predetermined concentration by evaporation, burning thecombustible organic constituents of the concentrated liquor in a furnacechamber under conditions yielding a dry ash having a high percentage ofcaustic magnesia and combustion gases containing a low percentage ofsulphur dioxide and substantially all of the ash in suspension therein,separating ash from the combustion gases, then passing unconcentratedliquor from the first washing stage into intimate contact with thecombustion gases for the recovery of additional ash from the gases andpartial evaporation and neutralization of the unconcentrated liquor,completing the neutralization of the partially neutralized andevaporated liquor by the addition of caustic magnesia thereto vprior tofurther evaporation thereof, mixing recovered ash with liquor from asubsequent washing stage to form an alkaline aqueous suspension, andpassing the ash suspension through a gas absorption chamber in contactwith the combustion gases to recover sulphur dioxide.

3. The method of treating the residual liquor resulting from thedigestion of cellulosic ibrous material in a relatively pure magnesiumbase sulphite cooking liquor which comprises evaporating the liquor to apredetermined concentration in a multiple-effect evaporator, burning thecombustible organic constituents of the concentrated liquor in a furnacechamber in suspension there- 'in under conditions yielding an ash having'a high percentage of caustic magnesia, mixing recovered ash withresidual liquor to neutralize the residual liquor before evaporation,and supplying ammonia to the vapor section of the evaporator to minimizecorrosion therein due to acid constituents of the vapor generatedtherein.

40 4. The method of treating the residual liquor resulting from thedigestion of cellulosic fibrous material in a relatively pure magnesiumbase sulphite cooking liquor and separation from the pulp in a pulpwashing system which comprises concentrating the liquor by evaporation,burning the combustible organic constituents of the concentrated liquorin a furnace chamber in suspension therein under temperature conditionsand for an interval sufficiently brief to obtain a dry ash having a highpercentage of caustic magnesia and combustion gases containing a lowpercentage of sulphur dioxide and substantially all of the ashinvsuspension therein, separating ash from the combustion gases, thenpassing unconcentrated liquor into intimate contact with the combustiongases for the recovery of additional ash from the gases and partialevaporation and neutralization of the unconcentrated liquor, completingthe neutralization of the partially neutralized and evaporated liquor bythe addition of caustic magnesia thereto prior to further evaporationthereof, and treating the separated ash to form fresh magnesium basecooking liquor.

5. A cyclic method of treating ligno-cellulosic material which comprisescooking the ligno-cellulosic material with a solution of magnesiumbisulphite containing a substantial excess of free sulphur dioxide,releasing gases containing free sulphur dioxide during thecooking'operation, separating the acid waste liquor from the treatedcellulosic material after completion of the cooking operation,neutralizing the acid waste liquor by the addition of caustic magnesia,evaporating and burning the neutralized waste liquor to produce an ashconsisting mainly of magnesium oxide and combustion gases containing alow percentage of sulphur dioxide, adding a mag-I nesium compound to thewaste liquor prior to its evaporation as required to make up chemicallosses. making a slurry of the magnesium oxide ash, absorbing thesulphur dioxide in said comi bustion gases in said slurry to reformmagnesium bisulphite to be used in a subsequent cooking op'- eration,making up losses of free sulphur dioxide from the system by fortifyingsaid reformed magnesium bisulphite with a gas resulting from the burningof a sulphurous material and absorbing free sulphur dioxide from thegases released during cooking in said reformed magnesium bisulphite, andreturning the reformed magnesium bisulphite containing the absorbedsulphur dioxide for further cooking of ligno-cellulosic material.

6. The process of treating the residual liquor resulting from thedigestion of cellulosic fibrous material in a relatively pure magnesiumbase sulphite cooking liquor which comprises concentrating the residualliquor to a concentration within the range of 45-70% solids, sprayingthe concentrated liquor into a high temperature furnace chamber, burningthe liquor so introduced therein to a dry unsintered solid residuecontaining a relatively high proportion of caustic magnesium oxide andto release sulphur dioxide, continuously removing the residue from thefurnace chamber by notation in the gaseous products of combustion insuch a brief period of time that the dwell of the residue within thecombustion zone is insuillcient to effect dead-burning of the magnesiumoxide in the residue, returning a portion of the residue removed to theresidual liquor to neutralize the liquor before concentration, treatinganother portion of the residue removed with water to produce an alkalineliquor containing the recovered magnesium compounds, and absorbingsulphur dioxide from the gases after leaving the furnace chamber bycontact with the alkaline liquor thus produced.

7. The process of treating the residual liquor resulting from thedigestion of cellulosic fibrous material in a pure magnesium bisulphitecooking liquor which comprises concentrating the residual liquor,continuously spraying the concentrated liquor into a high temperaturefurnace chamber, dehydrating and burning the liquor so introducedtherein while maintaining a reducing atmosphere and a furnacetemperature below the fusion temperature of the non-combustibleconstituents of the liquor to yield a dry solid residue containing a.relatively high proportion of magnesium oxide and to release sulphurdioxide. continuously removing the residue from the furnace chamber bygas flotation, separating the residue from the gases after leaving thefurnace chamber, returning a portion of the separated residue to theresidual liquor to neutralize the liquor before concentration, treatinganother portion of the separated residue with water to produce analkaline liquor containing the recovered magnesium compounds, absorbingsulphur dioxide from the gases after leaving the furnace chamber bycontact with the alkaline liquor produced to make fresh magnesiumbisulphite cooking liquor, and replenishing any sulphur and magnesiumlosses in the system by the addition of magnesium sulphate to theconcentrated residual liquor before burning.

8. The process of treating residual liquor resulting from the digestionof cellulosic fibrous material in a relatively pure magnesium basesulphite cooking liquor which comprises concentrating the residualliquor, introducing the concentrated liquor into a primary combustionand reducing zone, burning the concentrated liq uor so introducedtherein while maintainingv a temperature and atmosphere therein'yieldinga dry ash containing a high percentage of reduced magnesium compoundsand to release sulphurous gases, withdrawing the ash from the primarycombustion and reducing zone through a secondary combustion zone byflotation in the combustion gases, supplying combustion air slightly inexcess of the theoretical combustion requirements and proportioning thesame between the zones to provide a strongly reducing atmosphere in theprimary combustion and reducing zone and to provide a slightly oxidizingatmosphere in the secondary combustion zone suilicient to complete thecombustion in suspension of any unburned combustibles in the combustiongases, recovering the sulphurous gases released by contact with anabsorbing liquid including recovered ash, and adding free sulphurdioxide to the sulphited liquid to form fresh magnesium bisulphitecooking liquor. l

9. The process of treating residual liquor resulting from the digestionof cellulosic fibrous material in a relatively pure magnesium bisulphitecooking liquor which comprises concentrating the residual liquor,introducingl the concentrated liquor into a primary combustion andreducing zone in a iinely divided spray, burning the concentrated liquorso introduced therein in suspension while maintaining a temperature andatmosphere therein yielding a dry ash containing a high percentage ofreduced magnesium compounds and to release sulphurous gases, withdrawingthe ash from the primary combustion and reducing zone through asecondary combustion zone by flotation in the combustion gases,

supplying combustion air slightly in excess of the theoreticalcombustion requirements and proportioning the same between the zones toprovide a. strongly reducing atmosphere in the primary combustion andreducing zone and to provide a slightly oxidizing atmosphere in thesecondary combustion zone sufllcient to complete the combustion insuspension of any unburned combustibles in the combustion gases,recovering' the sulphurous gases released by contact with an absorbingliquid including recovered ash, filtering the sulphited liquid producedto eliminate any unburned carbon remaining, and adding free sulphurdioxide to the filtered liquid to form fresh magnesium bisulphitecooking liquor.

10. The process of treating the residual liquor resulting from thedigestion of cellulosic brous material in a relatively pure magnesiumbase sulphite cooking liquor and separation from the pulp in a pulpwashing system which comprises spraying the residual liquor into a hightemperature furnace chamber, burning the residual liquor so introducedtherein While maintaining a furnace temperature below the fusiontemperature of the non-combustible constituents of the liquor to yield adry unsintered solid residue containing a relatively high proportion ofcaustic magnesia and combustion gases containing a low percentage ofsulphur dioxide, removing substantially all of the residue produced fromthe combustion zone by flotation in the combustion gases in such a briefperiod of time that the dwell of the residue within the combustion zoneis insumcient to eiect dead-burning of the magnesia in the residue.mixing the solid residue vwith pulp washer nitrate to produce analkaline aqueous suspension, and passing the suspensionproduced througha gas absorption chamber in contact with the combustion gases to recoversulphur dioxide. l

11. The process of treating the residual liquor resulting from thedigestion of cellulosic iibrous brief period of time that the dwell ofthel residue e within the combustion zone is insuilicient to effectdead-burning of the magnesia in the residue, mixing a portion of therecovered residue with the residual liquor to neutralize the residualliquor before its evaporation, forming an alkaline aqueous suspension ofanother portion of the recovered residue, and passing the suspensionproduced through `a gas absorption chamber in contact with gaseousproducts of combustion from the furnace chamber.

12. The process of treating the residual liquor resultingfrom thedigestion of cellulosic fibrous .material in a relatively pure magnesiumbase sulphite cooking liquor and separation from the pulp in a pulpwashing system which comprises burning the combustible organicconstituents of the residual liquor in a furnace lchamber in suspensiontherein while maintaining a furnace temperaturebelow the fusiontemperature of the non-combustible constituents of the liquor to yield asolid residue containing a relatively high proportion of causticmagnesia and combustion gases containing a low percentage of sulphurdioxide, removing the residue produced from the combustion zone bynotation in the combustion gases in such a brief period of time that thedwell of the residue within ,the combustion zone is insumcient to eiIectdead-burning of the magnesia in the residue, forming an alkaline aqueoussuspension of the solid residue, passing the aqueous suspension producedthrough a gas absorption chamber in contact with the combustion gases torecover sulphur dioxide.'and controlling the amount of solid residuesupplied to the gas absorption chamber in response to variations in thesulphur dioxide content oi the combustion gases passing to the gasabsorption chamber.

13. The process of treating the residual liquor resulting from thedigestion of cellulosic fibrous material in a relatively pure magnesiumbase sulphite cooking liquor and separation from the pulp in a pulpwashing system which comprises spraying the residual liquor into a hightempera-- ture furnace chamber, burning the residual liquor sointroduced therein while maintaining a furnace temperature below thefusion temperature of the non-combustible constituents of the liquor toyield a dry unsintered solid residue containing a relatively highproportion of caustic magnesio. and combustion gases containing alow'percentage of sulphur dioxide, removing substantially all of theresidue produced from the combustion zone by flotation in the combustiongases -in such a brief period of time that the dwell of the resi-,- duewithin the combustion zone is insufficient to eiect dead-burning of themagnesla in the 'resi due, mixing the solid residue with pulp washerilltrate to produce an alkaline aqueous suenension, passing the aqueoussuspension produced through a gas absorption chamber in .contact withthe combustion gases to recover sulphur dioxide, and controlling theamount of aqueous suspension supplied to the absorption chamber inresponse to variations in the sulphurl dioxide vcontent of thecombustion gases passing to the absorption chamber.

GEORGE H. TOMLINSON;

